Switched power supplies have numerous applications in electronic devices and motor drives, and several basic types are well known to those skilled in the art. For purposes of illustration, the invention will be described in the context of a conventional DC-to-DC buck converter that accepts a DC input voltage and produces a lower DC output voltage. Buck converters are typically used in low voltage applications requiring high amounts of load current (e.g., 30 amps or more). It should be understood, however, that the invention is usable in other types of switched power supplies, for example, boost converters.
FIG. 1 illustrates a single phase buck converter 100 which includes a high-side switch 105, a low-side switch 110 connected to the high-side switch at a switch node 115, an output inductor 120 connected to the switch node 115, and an output capacitor 125 connected to the output inductor 120. High and low-side switches 105 and 110 may be power MOSFETS, IGBTS, or other bipolar transistors or other suitable devices which can be switched between a highly conductive state and a substantially non-conductive state.
In operation, gate drive signals for the high-side and low-side switches 105 and 110 are provided by a control circuit 130 to produce a desired output voltage across a load 135. For this purpose, control circuit 130 includes an oscillator and logic circuits which control the on and off times of the switches. Thus, when the high-side switch 105 is initially switched on, the low-side switch 110 remains off. This produces a voltage drop across the output inductor 120 of approximately (VIN-VOUT), which causes a current to build up in the inductor.
The high-side switch 105 is then turned off, and the low-side switch 110 is turned on. Since the inductor current cannot change instantly, it must flow through switch 110 which charges output capacitor 125. This causes the voltage (VOUT) across the output capacitor to rise.
Ultimately, as the high-side and the low-side switches 105 and 110 continue to be switched on and off at appropriate times, the voltage (VOUT) across the output capacitor 125 ultimately reaches a desired level, which typically, in the case of the buck converter, is lower than the input voltage.
Once the desired output voltage is reached, the switching on and off of the high-side and the low-side switches 105 and 110 continues, with the duty cycle, i.e. the relative on and off times of the switches, being controlled so that the output inductor 120 provides an amount of current equal to the current demand of a load 135 connected across the output capacitor 125. For this purpose, a suitable feedback regulation loop is provided. A circuit included in control circuit 130 receives a signal over a signal path 140 which is used to control the switching times of switches 105 and 110. A sensing device, of which signal path 140 is representative, may be responsive to the output voltage across capacitor 125 to provide so-called voltage mode control, or to the current through output inductor 120, to provide current mode control.
By properly controlling the duty cycle, the devices may be made to operate so no more and no less than the current demand of the load 135 is provided, and the voltage (VOUT) across the output capacitor 125 remains substantially constant at the desired output voltage with a desired degree of regulation.
Where the current demand of the load exceeds what can conveniently be provided by the circuit of FIG. 1, several such circuits can be combined to form a multi-phase DC-to-DC buck converter. A representative circuit topology for a multi-phase buck converter is shown in FIG. 2, designated generally at 200. This includes a plurality of interleaving output phases 205a, 205b, 205c, . . . , 205n, a multi-phase control circuit 210, and a feedback circuit (not shown) of any suitable design, as will be understood by those skilled in the art. Each output phase includes a high-side switch, a low-side switch, and an output inductor, as in the single phase buck converter 100 of FIG. 1.
The operation of converter 200 is generally the same as that of single phase converter 100. Thus, control circuit 210 periodically operates the output phases in a time-delayed sequence, with a duty cycle determined by the feedback signal, thereby sharing the current generation amongst the phases, and distributing heat generation experienced by the MOSFETS.
For buck converters as described above, the switching times of the high and low side MOSFETS are controlled by a PWM circuit. Typically, this includes an oscillator which generates a triangle wave and suitable logic circuitry which converts the triangle wave to a series of pulses according to an error signal representing the difference between a reference voltage and a voltage derived from the feedback signal. According to conventional practice, a fixed frequency oscillator is employed, and the duty cycle varies according to the value of the error signal. Alternatively, it is known to employ a variable frequency oscillator, with a fixed duty cycle.
Power converters that operate at a fixed switching frequency have desirable electrical noise characteristics. The amplitude of the modulation signal can use the entire common mode range of the control IC and not compromise the amplitude for variable frequency. The fixed frequency enables use of simple filters and blanking techniques to suppress any electrical noise emitted from the converter.
However, the selection of the fixed switching frequency involves a tradeoff of light load efficiency and transient response. A low switching frequency yields the best light load efficiency. A high switching frequency gives the best transient response. Thus, variable frequency operation has potential benefits.
Known techniques, however have associated disadvantages which have rendered them impractical up to now. Among the techniques which have been tried are constant on-time controllers and hysteretic controllers. The hysteretic controllers exhibit unacceptable noise levels because they rely on large output voltage ripple. The constant on-time controllers work well at light load but suffer from the transient delay problem described above.
Digital methods have also been proposed for multiphase converters that respond to a load transient by simultaneously turning on all power channels once the output voltage sags below a threshold. This method can be tuned for a full load step, but with a partial load step, this method turns on all of the power channels and builds up too much total current in the inductors. This causes the voltage to increase and overshoot the regulation value.
As noted above, fixed switching frequency converter is limited in its speed of response to a load transient. As an example, in a voltage-mode buck converter, the turn-on interval of the upper MOSFET starts with a CLK edge. The control generates one CLK edge every switching period so that the CLK frequency is equal to the switching frequency.
At the end of the on-time interval, the upper MOSFET turns off.
Assume that at this instant, a sudden load is applied to the output. The converter must wait for the next CLK edge before the next on-time interval. Meanwhile, the output voltage sags, as the load pulls current from the output capacitor. The output voltage continues to fall until the converter can ramp the inductor current to the new load current. The inductor current increases during the upper MOSFET next turn-on interval which starts at the next CLK edge. The response is delayed for the off-time interval which is equal to the CLK period less the turn-on interval. Therefore, converters with high switching frequency (and a short CLK period) respond faster than converters with low switching frequency.
The converter's efficiency is inversely proportional to the converter's power dissipation. The converter's power dissipation can be thought of in terms of conduction losses and switching losses. The switching losses are power dissipation that is related to the converter's switching frequency. The output inductor's core losses and the MOSFET switching losses are typical of switching power dissipation. These losses increase with increasing switching frequency. At light load, the output power and the conduction losses decrease, but the switching losses stay constant. The resulting efficiency is lower at light load.
The limitation on the speed of response to a load transient with a fixed switching frequency presents another problem in multi-phase converters. Because of the high side switches can not turn on until they receive their CLK signals, there is likely to be mismatch between the output voltages of the individual phases, with possible overloads on one or more phases.
It would therefore be desirable to have a control scheme for a switched power supply which exhibits the desirable noise characteristics of the fixed frequency drive and the fast response to load transients exhibited by a variable frequency drive.